The asteroid 2012 DA14, which will come within about 17,000 miles of Earth on February 15, is about half the size of a football stadium, and in a collision would generate an explosive energy equivalent to 2,500 kilotons of TNT. In comparison, the atomic bomb over Hiroshima that instantly killed more than 70,000 people released “merely” the equivalent of 17 kilotons of TNT.

Seventeen-thousand miles seems like plenty of room, but in cosmic terms, it's an awfully close shave. “Remember, the Earth is a moving target, traveling around the sun at 65,000 miles per hour,” former astronaut Ed Lu said in a public appearance at the Stanford Institute for Economic Policy Research earlier this month. “So [the asteroid] is missing us by only about 14 minutes.”

To be clear, the asteroid is not going to collide with Earth. But if it did, it'd have a devastating impact -- one that highlights Earth's vulnerability to a tough-to-detect mainstay of the cosmos: mid-sized asteroids capable of delivering nuclear-sized blasts. Comparable in size to the asteroid that destroyed 1,000 square miles of trees and reindeer in Tunguska, Siberia in 1908, 2012 DA14 would be very bad news in a direct collision with a populated area. Imagine a giant explosion in the sky, followed by a blast wave that would level buildings, knock the Golden Gate Bridge into the sea, and subject an area between San Francisco and San Jose to total destruction. A Spanish dental surgeon and amateur astronomer named Jaime Nomen first spotted 2012 DA14 last year –- hence the “2012” in its name –- so you might think that would give officials ample time to come up with an asteroid-deflection plan. But no. “With one year’s notice, there’s absolutely nothing we can do,” Lu said. “There’s no launch opportunity –- the asteroid is orbiting back around the sun. Had it been coming back to hit us, the only option would have been to evacuate. That’s not a good option.”

The good news: With enough warning -- preferably decades -- an asteroid headed for Earth could be deflected. Ramming a remotely controlled spacecraft against an asteroid to change the velocity by just millimeters per second can avert a collision with Earth. If, that is, we have at least 10 years notice before a collision. With less time, the change in velocity needs to be far greater. “The curve goes from millimeters per second to meters per second pretty quickly,” Lu told Popular Science. “The job rapidly goes from ‘easy- easy’ to almost impossible starting at about a decade.”

There are about one million asteroids larger than 40 meters that scientists consider “near Earth objects,” because their paths around the sun criss-cross the Earth's orbit. NASA’s near-Earth object office in Pasadena, California, reports that humans have spotted about 94 percent of the really large, civilization-ending near-Earth asteroids – in the 1- to 10- kilometer range, like the monster that wiped out the dinosaurs 65 million years ago – and concluded that none so far discovered will hit Earth in the next hundred years. But due to budgetary constraints, Lu points out that we have identified the orbits of only 1 percent of the still potentially dangerous medium-sized asteroids of at least 40 meters – like 2012 DA 14 or the Tunguska asteroid.

Though amateur and professional astronomers on Earth have spotted the NEOs that we do know about, there are limits to what terrestrial telescopes can accomplish. Telescopes only work at night, which prevents us from seeing asteroids approaching the planet from the inside of Earth’s orbit. Also, many asteroids are dark black and reflect less than 10 percent visible light, making them hard to spot from Earth. They do emit infrared light, but many infrared wavelengths do not make it through the Earth’s atmosphere. Lu has raised several million dollars toward a final goal of roughly $400 million through his B612 Foundation to launch a telescope called Sentinel into orbit near Venus. During a proposed 6.5 year mission, Sentinel will spot asteroids that cannot easily be identified from Earth. If successfully launched in 2018, Lu promises that Sentinel will spot about 500,000 NEOs, including 90 percent of all NEOs that are more than 140 meters, and 50 percent of the Tunguska-sized 40-meter rocks.

The only warning sign is a flash in the sky and a tidal wave.In making his fund-raising pitch, Lu likes to compare the asteroid threat to the risks we face every day. Our planet has about a 30 percent chance of getting hit by a Tunguska- sized 40 meter asteroid in the next 100 years -- compare that to the 23 percent chance an American has of dying of cancer. There’s about a 1 percent chance of getting hit by a 140-meter asteroid in the next century, which would unleash the power of 100 megatons of TNT -- twice as large as the largest nuclear bomb ever exploded, the Soviets’ Tsar Bomba detonation in 1961. As a comparison, a person has about a 1 percent chance of being killed in a car crash. And in the next 100 years, there is roughly a .01% percent chance of getting hit by a 1-kilometer or greater asteroid that would destroy all of human life on Earth. A 1-kilometer and up asteroid would blanket the hemispheres with enough dirt and dust to destroy several years of food growing season, leading to a Mad Max-like scenario in which survivors would quickly exhaust the world’s three-month food supply. As a comparison, any given American has about a .01% chance of dying in a plane crash. “I think the governments of the world are very good at confronting a threat that is quantified: real time, date, place,” Lu says. “When things are probabilistic? We’re just not good at that.”

Lu compares the Sentinel project to a safety-precaution against a small but real threat of disaster, like putting on a seat belt before driving. And he stresses that the asteroid problem can be easily handled –- so long as we know where the asteroids are well in advance. “If you don’t know where they are, the only warning sign is a flash in the sky and a tidal wave.”

Near Miss Asteroid 2012 DA14

1. Asteroids are cool

Whether they fly by Earth or not, asteroids are important remnants of the early solar system. They formed early in the solar system's history, and their compositions may hint at why our neighbors are so diverse, from rocky Mars to gaseous Jupiter. That's one reason scientists are increasingly interested in probing the humble surfaces of asteroids. In 2007, NASA sent its Dawn spacecraft to the asteroid belt to visit Vesta and Ceres. Vesta is rocky and Ceres is icy, and the differences between the two could help explain what happened to differentiate the bodies in our solar system after they formed.

2. It's happened before

Friday's flyby is record-breaking; skywatchers have never before recorded an asteroid of this size passing so close to Earth. Unrecorded close calls are another story. In 1908, a hunk of space rock about 150 feet (45 meters) in diameter screamed into the atmosphere near the Tunguska River in Siberia. The asteroid or comet fragment — about the size of the White House — broke up explosively in the atmosphere, leveling more than 800 square miles (1,287 square kilometers) of forest. [The 10 Greatest Explosions Ever]

Fortunately, the Tunguska Event happened in an extremely remote area, with the nearest human eyewitnesses miles away. But a rock like the one that caused the Tunguska Event, or like the one that will zip by Earth Friday, could level Washington D.C., and its suburbs, said Mark Boslough, a physicist at Sandia National Laboratories in New Mexico, who has used computer modeling to recreate the Tunguska impact.

3. It will happen again

This graphic shows 2012 DA14's path past Earth.CREDIT: NASA

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2012 DA14's flyby is a close one, but the asteroid itself is an old friend. Though first discovered in 2012, the space rock passes by Earth fairly frequently. It has an orbit around the sun roughly similar to Earth's, and zings by about twice per orbit. That means we'll be seeing 2012 DA14 again, although not as up-close. The next near-approach by the asteroid is scheduled for Feb. 15, 2046, according to NASA, when 2012 DA14 will pass about 995,000 miles (1.6 million km) from Earth. (Needless to say, astronomers have different ideas about "nearness" than most people.)

4. No, really — 2012 DA14 is one in (half) a million

Observing and measuring asteroids like 2012 DA14 is important because there are at least 500,000 of them out there. Less than 1 percent of these midsized near-Earth objects have been discovered, according to NASA.

Discoveries are made with optical telescopes, though once asteroids get close enough, scientists can use radar to better understand them. Astronomers of the La Sagra Sky Survey at the Astronomical Observatory of Mallorca in Spain discovered 2012 DA14 last February when the asteroid was 2.7 million miles (4.3 million km) from Earth.

Scientists at NASA's Near-Earth Object Program Office say that an asteroid the same size as 2012 DA14 approaches Earth this closely about every 40 years. Once every 1,200 years or so, one this size hits the planet. [See Photos of Asteroid 2012 DA14]

5. It helps us prepare

2012 DA14 won't hit Earth. But a close flyby like Friday's gives scientists the opportunity to think about what they'd do in the case of a once-in-a-thousand-year event, Sandia's Boslough told LiveScience.

For example, size estimates of asteroids are tentative until they reach radar range, Boslough said. Scientists would be able to pinpoint where an asteroid on a collision course with Earth would hit, but they wouldn't know with complete certainty how big it would be. Thus, evacuation orders might need to go out to larger areas than the best estimate of the size of the impact would suggest in order to be safe.

"For a lot of us, this is an opportunity to think about that, because we are going to have the answer when it does fly by," Boslough said of 2012 DA14. "What is the true size going to be relative to the estimates?"